Yes. The old mute was cut at an angle at the G string. Anyway, at the G string end, cut from midpoint along the end to say 5/8" along the neck edge (you are cutting the edge closest to the nut, and the cut is perpendicular to the top of the bass). But remember to cut with the adhesive up because that's the way it goes in the ashtray.
You and Lokosh are on the right track. The keys to mechanical muting:
1. The Fender Precision Bass bridge ashtray. Only this method presses the foam into the string from the top. This means that as you fret, the longer the string length, the less the influence of the rubber, but the deeper the rubber presses into the string in compensation.
2. Stiff foam. I'm going to say I recall the strip being 1/2" wide and a little thicker, maybe 9/16". If anyone reading this has one, please measure it for our reference, and also how far from the open end of the ashtray the sponge is placed. I'm looking at
Permatite Dortite open-cell natural sponge rubber adhesive-backed gasketing, but I won't know until I get some.